Field of the Invention
The present invention relates to a communication device using an adaptive antenna which is suited to be the wireless base station of a mobile communications system or the like.
The next-generation mobile communications system named IMT-2000 is required to deliver not only voice communications services, had also video and other relatively large-volume data communications services. Because of these demands, as more advanced technology for increasing the system capacity of the wireless base stations, adaptive antennas (adaptive array antennas), which can improve the SIR (signal-to-interference ratio) of each user signal and increase the system capacity, have become strong candidates.
These adaptive antennas consist of a plurality of antenna elements provided at the base station of the mobile communications system, and arbitrary weights, (amplitude, phase) are applied to the signals input to the respective elements to perform beam formation in the desired direction. However, it is necessary to be able to control the weights applied to the antenna branches so that portions of the beam with a high gain are directed to the desired user (the user to be communicated with) and portions of the beam with low gain are directed to interfering users (users not to be communicated with).
Description of the Prior Art
FIG. 12 shows a conventional example of a configuration of a communicative device with an array antenna. In the figure, 1 is an array antenna consisting of a plurality of antenna branches (antenna elements). 2 is a duplexer, which is used to obtain isolation of a transmit/receive, path in the case that one antenna branch is used for both transmitting, and receiving, 3 are weighting multipliers 3, and when an adaptive array antenna (AAA) is used in the uplink, these weighting multipliers 3 multiply the weights by the uplink signals of each antenna branch. 4 is an adder that adds the outputs of these weighting multipliers 3.
5 is the adaptive processor (AAA weighting block) for the uplink, and this adaptive processor 5 calculates the weights of each antenna branch based on the uplink signals of each antenna branch, the combined signal from the adder 4 and an arbitrary reference signal set. The weights of each antenna branch calculated by the adaptive processor 5 are provided as input to weighting multipliers 9 corresponding to each antenna branch.
11 is a data generator, in which data generation is performed according to the coding and frame format required, and the data thus generated is branched through a signal splitter 10 and provided as input to the respective weighting multipliers 9, where it is multiplied by the weights from the adaptive processor 5. The output corresponding to each antenna branch (user signals) is multiplexed with the user signals in the same cell or the same sector for each branch by the user signal multiplexers 12, passes through the duplexer 2 and is provided as output from the array antenna 1.
In the system as described above, on the downlink, particularly in the case of FDD (Frequency Division Duplex) wherein the frequencies are different on the uplink and downlink, as shown in "The Effect of Interference Suppression in Forward Link by Adaptive Array Antenna Transmitting for W-CDMA Mobile Radio" (RCS 98-72), adaptive control is performed on the uplink but transmission is performed on the downlink using exactly the same adaptive weightings as those generated for the unlink, but the beam shape of an array antenna has properties that vary depending on the frequency, so the afore-described, method has a limitation in that it can be used only under conditions wherein the difference between the transmit frequency and the receive frequency is no more than roughly 10%.
In this case, since the weights of the uplink are used for the downlink in the prior art system, when the difference between the receive frequency and the transmit frequency is large in the case of FDD, the high-gain portion of the beam may not necessarily be directed in the desired user direction, and similarly there is no guarantee that the low-level beam is directed in the interfering user directions. This tendency worsens particularly in the case in which the frequency difference exceeds 10%, leading to deterioration of characteristics.
In addition, in the case of simultaneous communications by a number of users in excess of the degrees of freedom (N-1) of the antenna (number of antenna elements: N) such as in CDMA, the efficient improvement to characteristics is not feasible.